I managed to upload a text file through the web gui and managed to retrieve it with the tftp get command. I didn't manage to upload a extreme switch firmware through the web gui maybe there is a size restriction.

You don't have to use winscp, anything that supports sftp should be fine, cyberduck and transmit (osx only) works fine for me.

Uploading files to tftpboot directory did the trick…so there must be a file size limit in the web gui.

I do have the same problem here. While I managed to tweak PFsense so I can upload via gui a IOS-XE bin, now I discovered I'm not able to "put" files on the TFTP Server. This is very inconvenient. While you're on your router and trying to backup the startup conf on your tftp you'll not be able to do it. I'll dig more and come back.
Cheers,
N